1. Field of the Invention
The present invention relates to a thin film type longitudinal magnetic recording medium and a method of manufacturing the same.
2. Description of the Related Art
A thin film type longitudinal magnetic recording medium is advantageous over the conventional magnetic recording medium of coating type in that the magnetic layer can be made thinner and that a high coercivity and a high saturated magnetization can be obtained easily. Thus, the thin film type longitudinal magnetic recording medium is adapted basically for the high density recording. Particularly, where Co-based alloys are used for forming the magnetic layer, the saturated magnetization is greater than in the use of magnetic oxide, making it possible to increase the head terminal output.
The magnetic properties of a longitudinal magnetic recording medium comprising a magnetic layer formed of a Co-based alloy such as Co--Pt alloy depend greatly on the crystal structure and crystal orientation of the magnetic layer. Also, the crystal structure and the crystal orientation of the magnetic layer are greatly dependent on the film-forming method and the film-forming conditions. Thus, in many cases, it is impossible to ensure the properties required for the longitudinal magnetic recording medium by simply defining the composition of the magnetic layer.
In the conventional longitudinal magnetic recording medium comprising a magnetic layer consisting of a Co-based alloy, a particular film construction and a film-forming method are employed in order to control the crystal structure and the crystal orientation of the magnetic layer. For example, a magnetic recording medium prepared by successively forming on a substrate an underlayer consisting of Cr and a magnetic layer consisting of a Co-based alloy which may possibly contain a third element is described in (Opfer et al., "Thin-Film Memory Disc Development", Hewlett-Packard Journal, November, 1985, pp.4-10). The Cr underlayer used in this prior art is intended to control the coercivity of the magnetic layer. However, use of the underlayer results in an increased manufacturing cost of the magnetic recording medium.
Published Examined Japanese Patent Application No. 63-13256 discloses another prior art. In this prior art, a magnetic layer consisting of a Co--Pt alloy or a ternary alloy such as Co--Pt--Ni is formed on a substrate by sputtering under an inert gas atmosphere containing 35 to 70% of N.sub.2, followed by a heat treatment so as to manufacture a desired magnetic recording medium. The method of this prior art certainly permits improving the coercivity of the magnetic layer. However, requirement of the heat treatment after formation of the magnetic layer leads to a high manufacturing cost of the magnetic recording medium.
An additional prior art is disclosed in Published Unexamined Japanese Patent Application No. 01-144217. In this prior art, a magnetic layer consisting of a Co--Pt alloy or a ternary alloy such as Co--Pt--Ni is formed on a substrate by sputtering under an inert gas atmosphere containing less than 1% of N.sub.2 or O.sub.2. This method permits improving the coercivity of the magnetic layer. In addition, the manufacturing cost of the recording medium is not increased because a Cr underlayer need not be formed and the heat treatment after formation of the magnetic layer need not be employed in this prior art.
On the other hand, it is important to improve the corrosion resistance of a metal magnetic layer because the metal magnetic layer is corrosive. The corrosion resistance of the Co--Pt alloy noted above can be markedly improved by adding Cr to the alloy. However, the magnetic properties are markedly deteriorated if a magnetic layer consisting of a Co--Pt--Cr alloy containing a large amount of Cr is formed by the conventional method. When the Cr content is small, the magnetic properties of the magnetic layer are changed depending on the type of the sputtering apparatus used even if the same sputtering conditions are set for forming the magnetic layer. In short, it is difficult to obtain desired magnetic properties. In particular, where the magnetic layer is formed at a high speed, it is difficult to obtain desired magnetic properties. To reiterate, where it is intended to obtain a Co--Pt--Cr alloy magnetic layer having a high corrosion resistance by the conventional method without using a Cr underlayer and without employing a heat treatment after formation of the magnetic layer, it is substantially impossible to obtain a magnetic layer having magnetic properties adapted for a longitudinal magnetic recording medium depending on the sputtering apparatus used.
The magnetic properties of the Co--Pt--Cr alloy magnetic layer are deeply related to the easy axis of magnetization of the alloy, i.e., orientation of the [0001] axis (C-axis) of the hexagonal close packed (HCP) phase. In a magnetic film of deteriorated magnetic properties, the C-axis is oriented in a direction perpendicular to the film surface, resulting in a small squareness.
Presently, it is impossible to theoretically explain how the material, sputtering apparatus and sputtering conditions are related to the crystal orientation of the magnetic layer, making it impossible to design the sputtering apparatus based on the complete analysis of the magnetic properties of the magnetic layer to be formed.